Hearing aids are often manufactured with an acoustic sensor (i.e. a microphone) as well as a magnetic sensor (i.e. a tele-coil). The acoustic sensor is used as the principal sensor for sensing an input acoustic signal that contains acoustic information which may comprise audio information (i.e. speech, music or other important sounds such as alarms, warnings, etc.). The magnetic sensor is an alternate sensor that is used in certain situations for sensing an input magnetic signal that contains magnetic information that is in many instances similar to the audio information. Use of the magnetic sensor can be beneficial in various situations.
For instance, it is common to install magnetic loop systems in classrooms to improve the comprehension of audio information for hearing impaired students. The magnetic loop system comprises a wire that is placed in the baseboard of a room such as a classroom. In this case, an instructor speaks into a microphone which transduces the instructor's speech and provides an electrical signal to the magnetic loop which radiates a corresponding magnetic signal, having magnetic information which is similar to the audio information corresponding to the original speech signal, to people who are sitting in the room. Advantageously, the magnetic signal, which is an input for the magnetic sensor of the hearing aid, will not contain the acoustic background noise that is picked up by the acoustic sensor of the hearing aid.
In another example, it is well known that most telephones utilize magnetic fields to vibrate the receiver diaphragm in the telephone earpiece to produce an acoustic signal with audio information. The magnetic fields contain amplitude and frequency components that are similar to the audio information. Accordingly, the magnetic fields can be used as a magnetic signal with magnetic information that is similar to the audio information. However, the magnetic signal will not contain the acoustic background noise that is typically added to the acoustic signal by the environment after the receiver produces the acoustic signal. Therefore, the magnetic signal can be used to assist hearing aid users with telephone communication in noisy surroundings. In addition, the use of the magnetic signal from the telephone receiver as an input to the hearing aid prevents acoustic feedback from occurring because, in this case, the input signal to the hearing aid is magnetic while the output signal from the hearing aid is acoustic and there is no acoustic coupling between these signals.
Most prior art hearing aids provide both an acoustic sensor and a magnetic sensor but require the hearing aid user to manually switch between a microphone mode, in which the hearing aid processes the acoustic signal sensed by the acoustic sensor, and a tele-coil mode, in which the hearing aid processes the magnetic signal sensed by the magnetic sensor. Accordingly, when the hearing aid user enters an environment with a magnetic loop or the hearing aid user talks on the telephone, the hearing aid user needs to switch the hearing aid from the microphone mode to the tele-coil mode. Likewise, when the hearing aid user leaves the magnetic-looped environment or hangs up the telephone, the hearing aid user needs to switch the hearing aid to the microphone mode. Unfortunately, manual switch operation can be cumbersome. Moreover, engaging a switch in a hearing aid that is worn within the ear canal is usually difficult, and at times, impossible.
The magnetic receiver in a telephone usually contains a permanent magnet, and consequently there will be a permanent (DC) magnetic field in the vicinity of the telephone receiver. Accordingly, some prior art hearing aids that provide both microphone and tele-coil input modes use a magnetic reed switch that closes in the presence of a DC magnetic field to automatically switch between microphone and tele-coil inputs. However, the automatic switching only works when the DC magnetic field is sufficiently strong to actuate the magnetic reed switch. Many modern telephones and cell phones do not produce a permanent magnetic field of sufficient strength to actuate a magnetic reed switch. In addition, there may be occasions in which the hearing aid user is in an environment in which there is a strong magnetic field but the magnetic field does not contain any desired information that corresponds to audio information. In this case, a hearing aid using a magnetic reed switch will automatically switch to the tele-coil mode but the hearing aid user will not hear any useful signals.
Loop systems do not generate a DC magnetic field, and a reed switch will not be activated when a loop system is encountered. However, all loop systems and many telephones do produce alternating magnetic signals, and it is advantageous for a magnetic detection system to be sensitive to such alternating magnetic signals.